Frequency converters, more commonly called "mixers", are employed in communication receivers. They are used for converting an RF signal, transmitted over the air medium, to a lower frequency signal suitable for demodulation in a receiver. Double-balanced Metal-Semiconductor Field Effect Transistor (MESFET) mixers are frequently used in RF receivers. Double-balanced mixers are well-known to those skilled in the art and the reader is invited to consult "Microwave Mixers", Second Edition, Norwood, Mass., Artech House, 1993, pp. 333-369, by S. A. Maas for further information on this subject. This document is hereby incorporated by reference.
One of the parameters of a mixer is its linearity, which can be defined as the level or correlation between the output signal and the input signal. Such correlation is typically a linear relationship between the output signal and the input signal. A high level of linearity allows to reduce the amount of distortion in the output. Linearity is important since it defines the dynamic range of a receiver. Considerable effort has therefore been expended to improve the linearity of mixers.
A common technique used to quantify the linearity of a communications receiver is to specify its Input Third Order Intercept Point (IP3). This technique uses two closely spaced (in terms of frequency) sinusoidal tones applied to the RF input of the receiver. Non-linearities in the receiver circuitry give rise to harmonics in the output signals corresponding to the two sinusoidal tones. Most of these harmonics are far outside the passband of the receiver, so are of little concern. However, the third order terms comprising the second harmonic of one tone and the fundamental frequency of the other tone will fall within the passband, and can therefore cause interference that can not be filtered. It is important to reduce the non-linearities that generate these third order terms to reduce interference and expand the dynamic range of the receiver. IP3 therefore provides a good evaluation of the linearity of a receiver. It follows that higher values of IP3 (measured in dBm) results in a more linear receiver.
In down-converting MESFET mixers, three inputs are required. One input is the RF input signal and the second input is a Local Oscillator (LO) signal. The mixer combines these signals and generates an output at a lower intermediate frequency. Essentially, the RF input is multiplied by an LO frequency signal to produce a lower intermediate frequency (the output). At the first two inputs and at the output, balanced/unbalanced transformers (baluns) are used to convert the signals from single ended to differential. This architechture is known to improve IP3 and port to port isolation. Also, in the LO balun, a center tap is used and allows a Direct Current (DC) bias (the third input) to be applied to the gates of the MESFETs in the mixer. The DC bias is used as a level shifter and shifts the peak LO voltage level down so that the gate does not go into forward conduction. For more information on this topic, the reader is invited to consult the following references describing the DC bias: Maas, S. A., "A GaAs MESFET Mixer with Very Low Intermodulation", IEEE Transactions on Microwave Theory Techniques, MTT-35, 1987, pp. 425-429; Pavio, A. M. et al., "Double Balanced Mixers Using Active and Passive Techniques", IEEE Transactions on Microwave Theory Techniques, MTT-36, 1988, pp. 1948-1956; and, Weiner, S., Neuf, D., Spohrer, S., "2 to 8 GHz Double Balanced MESFET Mixer with +30 dBm Input 3rd Order Intercept", 1988, IEEE MTT-S Digest, pp. 1097-1100. The above documents are hereby incorporated by reference.
The linearity of a mixer is a function of the LO power. Typically, linearity increases as the LO signal power level increases. An on-chip LO buffer amplifier is routinely used to generate the high LO drive required for optimal performance. However, when the LO drive extends beyond a threshold value, the gate current of the MESFETs increases rapidly and the mixer linearity rapidly decreases as the gate-to-channel junction goes into forward conduction. This phenomenon is described at page 342 of "Microwave Mixers", Second Edition, by S. A. Maas referred to earlier.
Due to manufacturing process variations in electronic components, there is often a wide variation in the LO power that is applied to the mixer. Against this background, it appears that it would be beneficial to operate the mixer with as large a LO signal power level as possible thereby enhancing linearity and, ultimately, the dynamic range of the communications receiver. Improvements in mixer linearity and in receiver dynamic range result in reduction in the bit error rate and a wider range of operating conditions.